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Using a sensitive new imaging instrument on NASA's Cassini spacecraft,
researchers have discovered a large and surprisingly dense gas cloud,
sharing an orbit with Jupiter's icy moon Europa.

Stretching millions of miles around Jupiter, the donut-shaped cloud,
known as a "torus," is believed to result from the uncommonly
severe bombardment of ion radiation the jovian giant sends toward
Europa. That radiation damages Europa's surface, kicking up and
pulling apart water-ice molecules and dispersing them along the
moon's orbit into a neutral-gas torus with a mass of about 60,000
tons.

The cloud's mass indicates the intense radiation Europa faces has
more severe consequences than scientists thought, says Dr. Barry
Mauk of Johns Hopkins University Applied Physics Laboratory (APL),
Laurel, Md. Mauk heads the laboratory's research team whose findings
appear in the Feb. 27 issue of the journal Nature. The mass also
shows that Europa, in an orbit some 671,000 kilometers (416,000
miles) from Jupiter, wields considerable influence on the magnetic
configuration around the giant planet.

"Surprisingly, Europa's gas cloud compares to that generated
by the volcanically active satellite Io," Mauk said. "But
where Io's volcanoes are constantly spewing materials, mostly sulfur
and oxygen, Europa is a comparatively quiet moon, and the gas we
see is a direct consequence of its icy surface being bombarded so
intensely," he said.
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"By acting as both a source and a sink of charged radiation
particles, the dense gas torus gives Europa much greater influence
than was previously thought on the structure of, and energy flow
within, Jupiter's huge space environment, its magnetosphere,"
Mauk said.

The APL team studied images of Jupiter taken in late 2000 and early
2001 with the laboratory's Ion and Neutral Camera on NASA's Cassini
spacecraft, now in route to Saturn. Mauk says this is the first
substantial discovery made at an extraterrestrial planet using an
innovative technique known as energetic neutral atom imaging.

"Planetary magnetospheres glow with energetic neutral atoms,
much like a red-hot piece of iron glows with photons of light, and
such neutral-atom glows can be remotely imaged," Mauk said.
"To this point, no instrument has imaged that activity beyond
Earth's magnetosphere. Energetic neutral atom imaging makes visible
the three-dimensional structure of planetary space environments,
which, until recently, were invisible to remote imaging techniques,"
he said.

Research team members at the APL and co-authors on the Nature paper,
"Energetic neutral atoms from a trans-Europa gas torus at Jupiter,"
include Dr. Donald Mitchell, Dr. Stamatios Krimigis, Dr. Edmond
Roelof and Dr. Christopher Paranicas. Krimigis, head of the Space
Department at the laboratory, is principal investigator for Cassini's
Magnetospheric Imaging Instrument, which includes the Ion and Neutral
Camera.

The Magnetospheric Imaging Instrument built by the APL is one of
12 science instruments on the main spacecraft and one of six instruments
designed to investigate the space environments around Saturn and
its moons. Cassini will begin orbiting Saturn on July 1, 2004, and
release its piggybacked Huygens probe about six months later for
descent through the thick atmosphere of the moon Titan. Cassini-Huygens
is a cooperative mission of NASA, the European Space Agency and
the Italian Space Agency. NASA's Jet Propulsion Laboratory (JPL),
Pasadena, Calif., manages the mission for NASA's Office of Space
Science, Washington. JPL is a division of the California Institute
of Technology in Pasadena, Calif.

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